Whispering system 

Added a basic whispering system:
You can communicate with another player within a server via `whisper i text` or `chat whisper i`.

The text is rendered using the highlight color but maybe there should be another color (alpha?) to differ from other messages.
Also needs some handy way to select a player to whisper, would suggest to implement in the spectator menu and also add mute and friend option to this menu